Ggplot2 different alpha behaviour [duplicate] - r

Question

This question already has answers here:
geom_rect and alpha - does this work with hard coded values?
(4 answers)
Closed 2 years ago.
I recently upgraded to R version 3.2.3 and also to ggplot version 2.0.0.
Trying to upgrade some old code to the newer versions I encountered a weird behaviour with ggplot2 and its transparency settings.
Now my question is, is this a bug or a feature (if so, can someone enlighten me as to why its good to have it this way)? The result I want to have is (obviously) plot 2.
Say I plot a line and lay a rectangle with transparency over it like this:
library(ggplot2)
plot_data <- data.frame(x = 1:100, y = rnorm(100))
# Plot 1
ggplot(data = plot_data, aes(x = x, y = y)) +
geom_line() +
geom_rect(aes(xmin = 20, xmax = 50, ymin = -Inf, ymax = Inf), fill = "red",
alpha = 0.1) + ggtitle("Plot 1")
# Plot 2
ggplot() +
geom_line(data = plot_data, aes(x = x, y = y)) +
geom_rect(aes(xmin = 20, xmax = 50, ymin = -Inf, ymax = Inf), fill = "red",
alpha = 0.1) + ggtitle("Plot 2")
To my understanding plot 1 and 2 should be identical. However, I get the following plots:
Plot 1:
and plot 2:
Additionally, if I play around with the alpha-values (for example setting them to 0.01, I get the two following plots:
and

Answer 1

I believe that calling geom_rect without a data parameter will effectively draw an individual rectangle for each row of the data.frame which is why the alpha is "working", but not quite as expected. I have not been able to replicate and get to parity/agreement between the methods, but as you noted, I think it is doing something along the lines of drawing either 100 individual rectangles, or 30 (the width of the rectangles; from 20 to 50) which is why alpha = 0.1 / 100 and alpha = 0.1 / 30 gets you closer, but not quite matching.
Regardless, I would probably use annotate, as that better describes the behavior/result you are trying to achieve without issues and works, as expected, in both cases -- annotations will draw a single instance per geom:
ggplot(data = plot_data, aes(x = x, y = y)) +
# geom_rect(aes(xmin = 20, xmax = 50, ymin = -Inf, ymax = Inf, alpha = 0.1, fill = "red")) +
annotate("rect", xmin = 20, xmax = 50, ymin = -Inf, ymax = Inf, alpha = 0.1, fill = "red") +
geom_line() +
ggtitle("Plot 1")
ggplot() +
geom_line(data = plot_data, aes(x = x, y = y)) +
# geom_rect(aes(xmin = 20, xmax = 50, ymin = -Inf, ymax = Inf), fill = "red", alpha = 0.1) +
annotate("rect", xmin = 20, xmax = 50, ymin = -Inf, ymax = Inf, fill = "red", alpha = 0.1) +
ggtitle("Plot 2")